Musannaf Ibn Abi Shayba 2:220

Abu Bakr narrated to us, he said: Yahya ibn Adam narrated to us, he said: Sharik narrated to us, from Ya'la ibn 'Ata, from Mus'ab ibn 'Abd al-Rahman, from Ka'b, who said: Whoever gives the Adhan, seventy good deeds are written for him. And if he performs the Iqamah, then it is better.